1. Field of the Invention
This invention is concerned with improvements in and relating to fuel rods and assemblies for nuclear reactors, principally of the mixed oxide fuel (MOX) type and in particular, but not exclusively, for pressurised water reactors (PWR).
2. Present State of the Art
Operators of nuclear reactors throughout the world increasingly wish to run their reactors on a longer fuel cycle than that previously entertained. Eighteen or even twenty four month cycles are now preferred between reloads. Whilst such runs call for more expensive fuel to be provided in the first place, cost savings are made in terms of the overall power generation from the fuel cycle as the shut down period for re-fuelling occurs less frequently. A re-fuelling cycle can take up to six weeks during which time the reactor is off line. The procedure also involves an experienced and consequently expensive team to be involved on site. In addition, re-fuelling is normally followed by having to obtain regulatory approval before the reactor can be brought on line once more.
Where longer fuel cycles are employed, to achieve the desired level of activity and hence power output from the reactor at the end of the cycle, the fuel must provide a higher reactivity level to start with. To provide even output over the cycle despite this higher initial reactivity a form of neutron poison, such as boron-10, must be provided at start up to inhibit the reactivity at this stage. The poisons involved, which may be of a variety of types, commonly act as neutron absorbers and consequently depress the reactivity whilst present. Whilst these poisons are essential during the initial part of the cycle, towards its end they must no longer be present as this would defeat the object by lowering the reactivity at the end of the cycle too.
Control of reactivity has previously been addressed in a number of ways in conjunction with solely uranium fuel reactors.
Control has been provided in the past by providing poison materials in the reactor at start up and during the initial part of the cycle and subsequently physically removing them as the cycle progresses. Clearly such an arrangement is mechanically and methodically complex.
Other control techniques have involved the provision of discrete poison rods which are inserted into the guide thimbles of a fuel assembly. Thus the fuel material is provided in one or more grades in rods and the poison is provided in a separate set of rods. Unfortunately, the provision of such discrete rods in the right amount to provide the desired level of reactivity control at the beginning of the cycle and yet not interfere with the end of the cycle is very difficult indeed. In addition, even if the poisons burn out during the subsequent part of the cycle the overall assembly is effectively unbalanced where the poison rod used to be. This lack of balance can present itself as hot spots which are highly undesirable and can lead to stresses which deform the assembly in that region. Additionally, these "empty" spaces formerly occupied by the poison rods are not available for water passage and consequently the transfer rate is diminished.
In a further prior art system UO.sub.2 fuel rods only have been provided in conjunction with lower grade UO.sub.2 which contains the poison; 6 wt % poison in 1.8 wt % U.sub.235. The poison is provided as a part of the fuel or alternatively as a coating around the circumference of the pellets. Construction of such assemblies is complex and time consuming as different levels and contents are frequently employed for different parts of the reactor fuel cell. Even longitudinally, within a given rod the enrichment grade and poison presence may vary.
The prior art problems are particularly acute in MOX reactor cores as MOX itself is a bigger absorber of neutrons than UO.sub.2. As a consequence poison provision using discrete rods is even harder to control than in other fuel types as the burn out rates for poisons in such MOX reactors is lower. If combined UO.sub.2 poison rods are employed in such reactor systems a further problem occurs where the poison does eventually burn out due to the differential absorption properties of MOX and UO.sub.2. As a consequence power peaking around the formerly poison containing UO.sub.2 rod occurs at a later date. Once again, this can lead to undesirable hot spots within the core leading to undesirable stresses and potential melting of the fuel rods.
Similar problems occur where discrete poisoning is provided within fuel rods or pellets, for instance as a central core. Neutron access for the poison is poor in such cases and once burnt out gives significant variations with location.
Poison systems have not successfully been employed in MOX fuelled reactors to date. At present at least 3 plutonium grades are provided in the fuel rods.